git-commit-graph.txt: fix list rendering

git-commit-graph.txt: fix list rendering

The first list item follows immediately on the paragraph where we
introduce the list. This makes the "*" render literally as part of one
huge paragraph. (With AsciiDoc, everything is fine after that, but with
Asciidoctor, we get some minor follow-on errors.) Add an empty line --
with a list continuation ("+") -- to make the first list item render ok.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>